The present invention relates to a method of manufacturing a product using scan targets and, more particularly, to a method of manufacturing aircraft using removably secured scan targets.
The accuracy with which automated manufacturing equipment is able to work parts depends largely on the quality of the location and orientation information used with the equipment. For example, with poor location and/or orientation information about a part, the most state of the art manufacturing equipment will only be able to work the part in a marginal manner. Conversely, with precise location and orientation information, a marginal piece of equipment may be able to perform well.
Automated manufacturing processes requiring a moderate amount of accuracy do not call for particular part location and orientation information beyond knowing the part has been positioned in a standard place. For these processes, the accuracy of standard part positioning allows part working with sufficient accuracy. In these processes, for example, a part to be worked can be placed in a standard location in or near the applicable automated machine, for example by abutting a predetermined edge of the part with a predetermined edge of a work platform and the machine can be programmed to work the part in that standard location. In processes requiring only moderate accuracy, standard part placement and machine operation are precise enough to achieve desired results.
Automated manufacturing processes requiring a high level of accuracy call for part locating and/or orienting after the part has been positioned. Common part locating and/or orienting methods involve scanning the parts to be worked after standard positioning using a vision system scanning device such as a camera or laser vision system scanning device. In these methods, the scanning device acquires location and/or orientation information about the part by visually sensing predetermined part features. For example, the scanning device can be programmed to locate a particular outside edge and interior hole of the part. The machine then processes the information procured from scanning to locate and/or orient the part in a coordinate system. Next, the machine works the part based on the processed location and/or orientation information.
Some processes require automated machinery to work parts with a very high degree of accuracy. For example, very high accuracy is required where interchangeable hole patterns are being used. Interchangeable hole patterns are those made in product parts likely to be interchanged during the life of the product. For example, while most other parts of aircraft may not require changing, it may be determined that a particular door typically requires replacement at least once during the life of the aircraft. In this example, the mating characteristics of the door and the door mounting location of the plane must lie within tighter than standard tolerances. Although parts of aircraft are generally manufactured at or about the same time and often in the same plant, an aircraft and a replacement part therefor may be manufactured at different plants and far apart in time. For example, an aircraft manufacturer may outsource replacement part manufacture to a supplier. Although various errors in a process repeatedly performed in the same place and time may cancel each other out or aggregate within acceptable limits, a part made decades later at a different location is less likely to have these benefits. For example, errors in formation of a first part are more likely to have errors that correspond to complimentary errors made in a second part on the same machine on the same day. Although standard tolerance ranges may be from about three hundredths of an inch to about six hundredths of an inch, applications such as interchangeable hole patterns can require tolerances of about one hundredths of an inch or less. Some applications would benefit from tolerances as low as about three thousandths of an inch or less.
Most conventional part locating and orienting processes, rely on standard part placement and/or scanning part features. These processes are unable to work parts to a repeatable degree of accuracy better than about three hundredths of an inch. Because parts usually have some amount of dimensional error, it is important for a part locating and/or orienting process to accurately recognize such errors. Sources of part error vary depending on the type of part and part features involved. One source of error is unintended dimensional variation. Dimensional variation occurs when at least one relevant part feature is malformed. As an example, FIG. 1 shows a part 10 of an aircraft wing including a first edge 12 having hinge apertures 14 cut in it. The location of corners 16, 18 can be determined through scanning the profile of the wing 10 and entered into a data processor (not shown). In many systems, the location of the edge 12 is determined by interpolation based on the location of the corners 16, 18. However, according to the location of the corners 16, 18, the edge 12 is expected to lie on a straight line 20 connecting the corners. In some systems, the location of the edge 12 is determined by interpolating and perhaps extrapolating from information acquired about the location of a few predetermined points (not shown) on the edge. Depending on the location of the particular points and the location of any edge 12 malformation, the method of interpolating and extrapolating from identified points on the edge may also result in a misunderstanding of where the edge is actually located. Thus, when automated machinery makes holes 22 at a predetermined distance 24 from the edge 12 based on its expected location, the holes near the error end up being too close to the actual edge. Insufficient edge distances can lead to tear problems whereby a part can rip next to the inadequate edge distance during use. Conversely, if edge 12 included an error that caused the side to follow a wide line 26, the distance between the holes 22 and the edge near the error would be too large. Insufficient and oversized edge distances may cause part fit problems. In both situations (i.e., insufficient and oversized edge distances), process accuracy would not be improved by scanning a part feature 28 protruding from the edge 12 halfway between the corners 16, 18. That is because the feature 28 lies along the specification line 20 for the edge 12 and further leads the data processor to calculate the edge 12 is straight. Further, visual characteristics of product feature 28 may render it difficult to accurately and reliably determine its location and orientation by scanning. For example, its shape, size, and color qualities may be insufficient to be reliably scanned. As insufficient or oversized edge distance may cause part weakness and fit problems, post-scanning part working errors due to inaccurate product location and orientation information can lead to these and other manufacturing and operational problems.
Dimensional error can also result from paint or other coatings applied to a thickness outside of a specified thickness range. Another common source of dimensional error results from separation distances between mating parts being outside of specified limits. Error due to dimensional variations is compounded when a part comprises multiple components and/or connections such that the errors aggregate. Some manufactures attempt to increase accuracy by manually creating each hole or other working requiring highly accurate placement. For example, one method of manual part working uses a “blanket” template having precise openings therein corresponding to locations for work to be done on the product. In use, a worker lays the template over the part and drills holes, or otherwise works, through the openings in the template. Manually working parts is time consuming and labor intensive and, thus, costly. Further, manual part working often does not result in the desired precision. Another conventional strategy for increasing accuracy of part working is to implement more precise scanning and working machinery. However, more precise machinery is very costly and can still produce insufficient accuracy.